Nanotube enabled, gate-voltage controlled light emitting diodes

ABSTRACT

Embodiments of the invention relate to vertical field effect transistor that is a light emitting transistor. The light emitting transistor incorporates a gate electrode for providing a gate field, a first electrode comprising a dilute nanotube network for injecting a charge, a second electrode for injecting a complementary charge, and an electroluminescent semiconductor layer disposed intermediate the nanotube network and the electron injecting layer. The charge injection is modulated by the gate field. The holes and electrons, combine to form photons, thereby causing the electroluminescent semiconductor layer to emit visible light. In other embodiments of the invention a vertical field effect transistor that employs an electrode comprising a conductive material with a low density of states such that the transistors contact barrier modulation comprises barrier height lowering of the Schottky contact between the electrode with a low density of states and the adjacent semiconductor by a Fermi level shift.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of copending U.S.Application entitled “NANOTUBE ENABLED, GATE-VOLTAGE CONTROLLED LIGHTEMITTING DIODES” having Ser. No. 12/677,457, filed May 12, 2010, whichis the U.S. national stage application of International PatentApplication No. PCT/US2008/075866, filed Sep. 10, 2008, which claims thebenefit of U.S. Provisional Application Ser. No. 60/971,147, filed Sep.10, 2007, claims the benefit of U.S. Provisional Application Ser. No.61/085,670, filed Aug. 1, 2008, and is a continuation-in-part ofInternational Application No. PCT/US2007/072501, filed on Jun. 29, 2007,which claims the benefit of U.S. Provisional Application No. 60/817,521,filed on Jun. 29, 2006, all of which are hereby incorporated byreference herein in their entirety, including any figures, tables, ordrawings.

BACKGROUND OF THE INVENTION

Light-emitting transistors combine the visible light emission propertiesof light emitting diodes (LEDs) with the switching properties oftransistors. As driving elements, lateral- (e.g., field effecttransistor (FET)) and vertical- (e.g., static induction transistor) typetransistors have been proposed. Lateral-type transistors using astandard FET structure require a high drive voltage due to factors suchas a relatively long channel length, low luminance efficiency, and smallaperture ratio. Vertical-type transistors using an organic staticinduction transistor have relatively high currents and high speeds withlow operational voltages, but the fabrication of a fine gate structurehas been conventionally required to achieve a high on/off ratio.

In conventional nanotube network based field effect transistors, thenanotube network is directly contacted on two sides by metallic sourceand drain electrodes. To observe a significant gate induced modulationof the current through the nanotubes, between the source-drainelectrodes, the surface density of the conventional nanotube networkneeds to lie very close to its percolation limit. This is because thenanotubes of the networks of field effect transistors are a mixture ofsemiconducting and metallic nanotubes, while only the carrier density ofthe semiconducting nanotubes are appreciably modulated by the gatefield. If the nanotube surface density lies well above the percolationthreshold, there are numerous purely metallic nanotube current pathwaysbetween the source and drain electrodes. This results in substantialsource-drain current even when the gate field modulates thesemiconducting nanotubes to minimize their conductance (the “off”state). When the gate field maximizes the semiconducting nanotubeconductance (the “on” state), the overall source-drain conductance doesincrease. However, if the nanotube surface density is well above thepercolation threshold, the increase in the “on” state current is only afraction of the “off” state current. It is only when the nanotubesurface density is very near the percolation threshold and the greatmajority of what would otherwise be purely metallic nanotube currentpathways are interrupted by semiconducting nanotubes that the “on” statecurrent can be orders of magnitude greater than the “off” state current.

Thin film transistors (TFTs) provide the drive circuitry for present andemerging active matrix displays including liquid-crystal andorganic-light-emitting display technologies. The dominant activesemiconductor in these devices is amorphous silicon, however the promiseof inexpensive, solution based processing techniques, inkjet patterningand construction on flexible plastic substrates has focused muchresearch over the past 20 years on organic semiconductors asreplacements. There now exist a broad range of small molecule organicand polymeric compounds that have demonstrated transconductance.Unfortunately, the electronic mobilities of these compounds, which wereinitially about 5-6 orders of magnitude too low to be commerciallyuseful, remain about an order of magnitude too low. Such low mobilitycan be compensated for by bringing the source and drain electrodescloser together, reducing the semiconductor channel length (C_(L) inFIG. 7A), but that greatly raises the cost of patterning the devices,removing much of the motivation.

A new TFT architecture was disclosed in Ma et al., Appl. Phys. Lett.2004, 85, 5084, to circumvent mobility limitations of present organicsemiconductors. The device relies on an ultra thin (<20 nm) aluminumsource electrode that required careful partial oxidation. While theoptimized device exhibited ˜6 orders of magnitude current modulation,the low work function aluminum source electrode required an n-typeactive channel, restricting that device to the use of C₆₀ as the channelmaterial. Li et al., Appl. Phys. Lett. 2007, 91, 083507 disclosed theuse of the organic semiconductor pentacene; but requires an additional 7nm vanadium oxide layer atop the partially oxidized aluminum sourceelectrode. As forming a partly oxidized, ultra thin aluminum sourceelectrode is difficult to produce commercially, constrains the choice ofthe organic active layers, and is susceptible to electromigration;hence, limiting the device lifetime other modes of forming an electrodethat does not shield the gate field is desired. Thus, there remains aneed for a light emitting transistor that is easy and efficient tomanufacture and can use a simplified electronics drive scheme at lowoperational voltages, thereby requiring less energy consumption andproviding for a longer and more reliable device lifetime.

BRIEF SUMMARY

Embodiments of the invention relate to a light emitting transistor thatincorporates a gate electrode for providing a gate field, a firstelectrode comprising a dilute nanotube network for injecting charges(holes or electrons), an a second electrode for injecting acomplementary charge (electrons or holes, respectively), and anelectroluminescent semiconductor layer disposed intermediate thenanotube network and the electron injecting layer. For example, the holeinjection can be modulated by the gate field. The holes and electronscombine to form photons, thereby causing the electroluminescentsemiconductor layer to emit visible light.

Embodiments of the invention allow facile modulation of the electroniccontact-barrier across the junction between single wall carbon nanotubes(SWNTs) and two distinct organic semiconductors. Specific embodimentsrelate to two devices: a vertical field effect transistor and a verticallight emitting transistor. The vertical architecture, which is readilyfacilitated by the specific properties of the nanotubes, allows the useof low mobility semiconductors that would otherwise be consideredunsuitable for field effect transistors. For the light emittingtransistor, the gate control permits new pixel drive schemes andincreased device lifetimes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary embodiment of a device of the presentdisclosure.

FIG. 2 shows an atomic force microscopy (AFM) image of an exemplaryembodiment of a dilute nanotube network on top of a silicon wafer, witha scale of 1×1 micron.

FIG. 3 is an exemplary embodiment of a device of the present disclosureduring an experiment in which the gate field is provided by an ionicliquid and the semiconductor is p-doped silicon.

FIG. 4 shows the modulation in I_(SD) with the same gate voltage appliedto each nanotube network-silicon interface for two different sourcedrain potentials of 0.1 and 0.3 volts.

FIG. 5 is a partial cross sectional view of a second exemplaryembodiment of a device of the present disclosure.

FIG. 6 shows the transfer curve for the device of FIG. 5.

The figures may not be drawn to scale. Moreover, where directional terms(such as above, over, left, right, under, below, etc.) are used withrespect to the illustrations or in the discussion, they are used forease of comprehension only and not as limitations. The elements of thedevices may be oriented otherwise, as readily appreciated by thoseskilled in the art.

FIG. 7 shows standard prior art TFT (7A) device and a VFET (7B) device,with a specific device (7C) employing a nanotube network according to anembodiment of the invention, where the curved lines represent thepercolating nanotube network, along with the wiring diagram for thedevice. FIG. 7A shows that a short channel length, C_(L), in thestandard TFT requires tight patterning of the source and drainelectrodes, an issue circumvented in the VFET. The current in thestandard TFT scales with the channel width C_(W), while in the VFET thisscales with the overlap area between the source and drain electrodes,C_(A).

FIG. 8 shows transistor characteristics of hole only VFETs, where FIG.8A shows source-drain current as a function of gate voltage for bothmaterial systems. PF-9HK devices have noticeably larger hysteresis thanthe NPD devices. Arrows indicate the gate voltage sweep direction. FIG.8B shows output curves for the PF-9HK VFET, and FIG. 8C shows NPD VFETat the gate voltage specified adjacent each curve.

FIG. 9 shows the HOMO vs. the horizontal position x at different gatevoltages, V_(G)=−1V, −10V, and −20V taken at the vertical position y=1nm, where the interface between the gate oxide and the polymer channelis defined as y=0. The equilibrium Fermi level in both the polymerchannel and the nanotube contact is E_(F)=0 (horizontal dash-dot-dotline). Vertical arrows indicate the barrier height φ_(bp) at eachvoltage. The nanotube diameter is 5 nm with its center located at x=0and y=2.5 nm. The inset shows the simulated structure and thecoordinates, inside the nanotube (|x|<2.5 nm) the electron potentialenergy (the symmetric point of the p_(Z) orbital bands) is plotted.

FIG. 10A shows luminance versus drain voltage at the indicated gatevoltages for the VOLET stack illustrated in FIG. 10B.

DETAILED DISCLOSURE

Embodiments of the invention are directed to light emitting transistorsthat have a gating electric field to control electronic transportbetween a first electrode comprising a dilute nanotube network and anelectroluminescent semiconducting layer. The nanotube network includesopen spaces that allow for penetration of the gate field into anelectroluminescent layer. The gate field controls the current injectioninto the electroluminescent layer and thereby the visible light outputfrom the device. Suitable applications of this technology includelighting, displays and signs, for example.

A gating electric field is used to modulate the barrier to electronictransport between a dilute nanotube network and a semiconducting layerby controlling current injection into the semiconducting layer. If thesemiconducting layer is an electroluminescent material, the devicebecomes a gate-voltage controlled light emitting diode (GLED), alsotermed a light emitting transistor. FIG. 1 shows a device according toan exemplary embodiment of the invention. Such an embodiment includes atransparent substrate layer 101. Layer 102 is a transparent gateelectrode topped by an insulating, gate dielectric layer 103. The curvedlines drawn on top of the gate dielectric layer represent the firstelectrode comprising a dilute nanotube network 104. This is coated withthe electroluminescent material layer 105, followed by a secondelectrode 106 and a protecting electrode layer 107, where, for example,in many embodiments of the invention, the first electrode injects holesand the second electrode injects electrons. It should be understood thatdevices of alternate embodiments of the invention comprise a firstelectrode to inject electrons and the second electrode to inject holes.In general, embodiments of the invention are described herein for holeinjecting first electrodes and electron injecting second electrodes,although those skilled in the art can envision devices with the conversestructure.

In one embodiment, the nanotube network 104 is made electricallypositive with respect to the electron injecting layer 106 and protectingelectrode layer 107. Holes injected from the nanotube network 104 andelectrons injected from the second electrode 106 combine in theelectroluminescent layer 105 to produce photons. The high aspect ratio(length to diameter) of the nanotubes allows the network to beelectrically percolating (electrically interconnected), butsimultaneously dilute, thereby possessing substantial open spacesbetween the nanotubes. As used herein, a dilute nanotube network is onethat has sufficient open spaces to permit some contact between theactive layer, such as the electroluminescent material layer 105 and thegate dielectric layer 103. These open spaces admit penetration of thegate field into the electroluminescent layer 105 without the fullelectrostatic screening that would be caused by a contiguous electrodein place of the nanotube network. The gate field modulates the carrierdensity in the electroluminescent material 105 in the vicinity of thenanotubes, and on the nanotubes, thereby modifying the barrier to holetransport between the two materials. The gate field thus controls thehole injection into the electroluminescent layer 105 and therebycontrols the light output from the device.

Dilute nanotube networks that are electrically percolating are readilyfabricated by the method described in U.S. Patent ApplicationPublication No. US 2004/0197546 (the '546 application), which teachesthe fabrication of dilute nanotube networks that are electricallypercolating. FIG. 2 shows an atomic force microscopy (AFM) image of anexemplary embodiment of a dilute nanotube network 104 on top of asilicon wafer, including open spaces between electrically interconnectednanotubes. The fabrication method described in the '546 applicationpermits fine control over the surface density of the nanotubes making upthe network 104 so as to allow fine control of the fraction of openspace between the nanotubes. The nanotube surface density that optimizesdevice operation depends on the characteristics of theelectroluminescent material and is determined experimentally.

Modulation of hole-injection by the gate field of a contact barrier atthe nanotube semiconductor interface was demonstrated as shown in FIG.3, which is an exemplary device according to an embodiment of theinvention in which the gate field is provided by an ionic liquid ratherthan a conductive electrode with a gate dielectric layer, and where thesemiconductor is p-doped silicon. The device is symmetric about itsvertical midline so only one of each repeated element is labeled. Layer305 is a 600 nm insulating oxide layer that has been etched away fromthe middle portion of the p-doped silicon substrate layer 306. Palladiumelectrodes 304, which are pre-deposited onto the oxide layer, provideelectrical contact to the nanotube networks 301. The nanotube networks301 are draped across the contact electrode 304 and down onto the baresilicon 306 to provide intimate contact between the nanotubes 301 andthe silicon 306. Separate ionic liquid drops 302 saturate each nanotubenetwork 301. The nanotube network 301 on the left is designated as thesource and the network 301 on the right is designated as the drain. Thenanotube networks 301 on each side are not physically connected to eachother; they are separated at their point of closest approach byapproximately 1 cm. A small potential, V_(SD), is applied between thesource and drain networks (via the electrodes 304) and the currentI_(SD) measured. The current path goes through the silicon and thereforethrough the nanotube-silicon interface on each side. Separate powersupplies apply gate potentials to each interface between nanotubenetwork 301 and Si substrate 306 via the ionic liquid drops 302. Thesepower supplies connect electrically on one side to the nanotube networks301 (via Pd electrode 304) and on the other side to a counter electrode303 contacting only the ionic liquid 302.

FIG. 4 shows the modulation in I_(SD) with the same gate voltage appliedto each nanotube network-silicon interface for two different sourcedrain potentials of 0.1 and 0.3 volts. The gate voltages clearlymodulate the transport barriers across each interface. The logarithmiccurrent scale presents the magnitude of the modulation. In theembodiment of FIG. 3, the nanotube networks are thicker than shown inFIG. 2 and have a much larger surface density than shown in FIG. 2.Nevertheless, the porosity of the network 301 and the fluidity of theionic liquid gate electrode allow access to the nanotube network-siliconinterface on each side to enable the effect.

The current modulation described here and below should not be confusedwith that seen in conventional nanotube network based field effecttransistors. The nanotube networks of the present disclosure do not lieas close to the percolation limit as conventional nanotube networks forconventional nanotube network based field effect transistors. Forspecific embodiments of the subject invention, direct modulation of thenanotube network conductance contributes only a small fraction to theoverall modulation produced.

Another embodiment of the invention has device geometry as that in FIG.5, where the nanotube network has a surface density as exemplified inFIG. 2. FIG. 5 illustrates a second exemplary embodiment of a device ofthe present invention. In this case, the dilute nanotube network 503(represented by the curved lines) lies on top of a 200 nm thick,gate-dielectric SiO_(X) layer 502 on a p-doped Si substrate 501 thatacts as the gate electrode. The active semiconductor layer 504 is aelectroluminescent polymer, which, in an exemplary embodiment, ispoly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(9-hexyl-3,6-carbazole)](PF-9HK). The top contact is a platinum electrode layer 505. Thisdiffers from the device of FIG. 1 in that the Fermi level of platinumlies much closer to the highest occupied molecular orbital (HOMO) of thepolymer rather than its lowest unoccupied molecular orbital (LUMO).Without electron injection into the LUMO, the device is a so-called“hole only” device (transport in the polymer is restricted to the HOMO)and does not emit light, obviating the need for transparency of the gatelayer and substrate of FIG. 1. This construction is useful fordemonstrating the efficacy of the gate in modulating the contact barrieracross the nanotube-polymer interface.

FIG. 6 shows the transfer curve for the device of FIG. 5. The gatepotential induces a two order of magnitude change in the source-drain(nanotube-platinum electrode) current. The large gate voltage needed tomodulate the source-drain current is due to the relatively thick, 200 nmSiO_(X) gate dielectric layer.

Many embodiments of the invention include the use of a transparentsubstrate and a transparent gate electrode. Use of an electroluminescentsemiconductor, and a low work function cathode permits the device tofunction as a gate-voltage modulated light emitting diode. Suitabletransparent substrates include glass and plastic substrates, forexample. Suitable transparent gate electrodes include transparentconducting oxides such as indium tin oxide or transparent nanotubefilms, such as described in the '546 application, for example. Theelectroluminescent semiconducting material can be in the form of apolymer, an organic molecule, or a conventional solid statesemiconductor such as GaN, for example.

The work function of atmosphere exposed and/or acid purified carbonnanotubes places their Fermi level near −5 eV. Because this energy liescloser to the hole band, or HOMO, of most electroluminescent (EL)materials, embodiments of the invention are readily prepared where thedilute network is made from such nanotubes that provide hole injectioninto those materials. The gate field in the device of FIG. 1 thusmodulates the barrier to hole injection between the nanotubes and the ELmaterial. In exemplary embodiments, particularly suitable EL materialshave hole bands or HOMO levels that lie near the nanotube Fermi level.For states within 1-2 eV of their intrinsic Fermi level, the nanotubespossess a low density of electronic states. This means that the nanotubeFermi level can be shifted by the electron injection or withdrawalinduced by local electric fields due to the source-drain potential andby gating electric fields. Consequently, for gate induced modulation,exemplary embodiments of the device use EL materials with hole bands orHOMO levels within 1 to 2 eV of −5 eV (having HOMO levels between about−3 eV and about −7 eV).

Another consideration for the suitable electroluminescent material isthat its intrinsic carrier density not be particularly large. In thecase of a large intrinsic carrier density, the width of the depletionlayer that makes up the barrier to hole transport between the nanotubesand the EL material is thin, while a large density of carriers causesthe gate field to be screened from penetrating with appreciable depthinto the EL material. These effects can make the gate modulation in thecase of high-carrier-density EL materials small.

Moreover, the nanotubes can cause electrostatic screening of the gatefield from the region of the EL material directly above them (on theirside nearest the top contact). Such screening makes it difficult for thegate field to turn off a current channel directly above the nanotubesthat is normally open. In a specific embodiment, the current channelthat is normally closed via current pathways lying to the sides of thenanotubes, where the gate field has access, can be turned on. ELmaterials used in specific embodiments possess a HOMO that lies belowthe Fermi level of the nanotubes.

Suitable example EL materials include a vast number of molecular,polymeric and oligomeric pi-conjugated organic materials that have beensynthesized to have HOMO energy levels that vary from as high as −4.0 eVvs. vacuum for neutral poly(3,4-ethylenedioxythiophene) (PEDOT), to aslow as −6.7 eV for poly(dicyanothiophene). In many instances, the piconjugated polymers are both photo- and electro-luminescent asillustrated by the materials in Table 1. Some embodiments of theinvention incorporate EL materials having a HOMO energy range of −5.1 eVto −6.0 eV, which are considered relatively low lying HOMO materials. Inthese embodiments, the polymers and oligomers can be made relativelypure and air stable and the intrinsic carrier concentration are low,leading to low charge mobilities. Between −5.1 eV and −6.0 eV, there aremany accessible materials; subtle changes in organic structure can allowthe HOMO level to be tuned.

TABLE 1 HOMO values for a series of representative conjugated polymersand oligomers suitable for use in embodiments of the gate-voltagecontrolled LED. Methoxy/EtHxOxy poly(phenylene vinylene) −5.1 eV to −5.4eV (MEH-PPV) Oxadiazole pendant poly(phenylene vinylene) −5.3 eVOligo(9,9-di-n-octylfluorene-2,7-vinylene)s −5.3 to −5.5 eVPoly(4-4′-diphenylene diphenylvinylene) (PDPV) −5.4 eVPoly(9,9-dialkylfluorenes) −5.6 to −5.7 eV Poly(bis acetylidethiophenes) with BTD or −5.5 to −5.7 eV quinoxaline Diphenyloxadiazolependant polystyrenes −5.4 to −6.0 eV

In addition to polymeric materials, there are numerous small moleculeorganic emitters, including metal chelates, distyrylbenzenes, andfluorescent dyes, possessing characteristics suitable for the devicefunction in other embodiments of the invention. Representative materialsand their HOMO levels are listed in Table 2.

TABLE 2 HOMO values for a series of representative small moleculeorganics suitable for use in embodiments of the gate-voltage controlledLED. 5,6,11,12-Tetraphenylnaphthacene −5.4 eVBis(4′,6′-Difluorophenylpyridinato)- −5.8 eV4,9-bis-[4-(2,2-diphenyl-vinyl)-phenyl]-naphtho[2,3-c][1,2,5]thiadiazole4,4′-Bis′2,2′-diphenylvinyl′-1,1′-spirobiphenyl −5.9 eV Factris(2-phenylpyridine) iridium [Ir(ppy)3] doped into −6.3 eV a4,4′-N,N′-dicarbazole-biphenyl

Representative materials for the dielectric layer may include a broadrange of insulating ceramics, such as SiO_(X), Al₂O₃, Si₃N₄, Y₂O₃,Ta₂O₅, PbTiO_(X), AlTiO_(X), glasses, organic compounds such as Parylenepolymers, polystyrene, polyimide, polyvinylphenol,polymethylmethacrylate, fluoropolymers and self assembled monolayers andcombinations thereof.

Representative materials for the gate electrode may include metallicallydoped and un-doped transparent conducting oxides such as ZnO, In₂O₃,SnO₂, CdO, doped with metals such as Al, Sn, Y, Sc and Ga and anycombinations thereof. Representative materials may also include metalssuch as, Al, Au, Ag, Pt, Pd, Cd, Ni and Ta, as well as combinationsthereof, as well as doped semiconductors such as p or n doped Si, p or ndoped GaAs.

The representative materials above are representative materials forwhich the device can function and are, by no means, all inclusive.Specific embodiments of the invention, depending on the specific ELmaterial used, can incorporate an electron transport layer betweenlayers 105 and 106, and/or one or more hole transport layers betweenlayer 104 (the nanotubes) and the EL layer 105. For embodimentsincorporating hole transport layers between layer 104 and the EL layer105, the gate field modulates hole injection into the first holetransport layer rather than into the EL layer.

Advantages provided by the gate modulated hole injection into the lightemitter devices include, but are not necessarily limited to, improvedlifetimes and simplified electronic drive schemes. As high drive voltagebias is detrimental to emission lifetime of LED devices due toelectromigration and heating phenomena, the lowering of drive voltagebias due to gate field enhancement of the hole injection can improvelifetimes of embodiments of the subject devices of embodiments of theinvention. Furthermore, gate voltage induced hole injection providesanother degree of electronic control over the device operation, allowingsimplification of the electronics for turning pixels on and off fordisplay device applications according to embodiments of the invention.

A TFT is a field effect transistor (FET) in which a gate field inducescarriers in the active layer permitting current to flow between thesource and drain electrodes. FIGS. 7A and 7B compare, in schematic form,a conventional prior art TFT and the architecture of a TFT in accordancewith an embodiment of the invention. In contrast to the conventional TFTin which the source 701, active layer 702 and drain 703 are co-planarwith respect to the dielectric 704 and gate 705, the architecture shownin FIG. 7B stacks the source 711, active layer 712 and drain 713vertically relative to the gate dielectric 714 and gate electrode 715,hence the designation as a vertical field effect transistor (VFET). Forthe VFET architecture a continuous metal source electrode wouldcompletely screen the gate field from the active layer, hence anecessary requirement for its operation is that the source electrode bein some sense perforated, making it porous to the gate field. The sourceelectrode 711 shown as a regular grid in FIG. 7B is meant to convey thisidea but should not be taken literally. In an embodiment of theinvention, illustrated in FIG. 7C, the gate-field-porous sourceelectrode 721 is a network of single wall carbon nanotubes. Not shown isa contact pad along the periphery of the device that connects thenanotube source electrode to the power supply. The network can be adilute layer that is well above the percolation threshold. Note that forthe VFET the channel length C_(L) is simply the active layer 722 filmthickness, which can be made almost arbitrarily thin, without the needfor high resolution electrode patterning for an equivalent drain 723,gate dielectric 724 and gate electrode 725.

FIGS. 8A and 8B show the transfer (I_(DS) vs. V_(G)) and output (I_(DS)vs. N_(DS)) curves for embodiments of the invention, illustrated in FIG.7C, usingpoly[(9,9-dioctyl-fluorenyl-2,7-diyl)-alt-co-(9-hexyl-3,6-carbazole)](PF-9HK) as the active semiconductor layer and gold as the drainelectrode. Also shown in FIG. 8A is the transfer curve (output curve inFIG. 8C) forN,N′-di(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-1,4′-diamine) (NPD) usedas the active semiconductor layer. The gate voltage sweep modulates thecurrent in both channel layers by two orders of magnitude. The largesubthreshold slope is a consequence of the relatively thick SiO₂ gatedielectric used. In order to avoid gate leakage, a thicker dielectriclayer can be used, avoiding complications regarding the device function.The device characteristics can be improved as the dielectric is madethinner. In contrast to standard FETs, where the on-current scales withthe channel width (a linear dimension), current in the subject VFETscales with the overlap area between source and drain electrodes (alinear dimension squared). The on-current can also scale with thedensity of nanotubes in the source electrode (up to a limiting densityat which the gate field begins to be screened by the nanotubesthemselves).

Evident in the transfer curves in FIG. 8A is a large hysteresis, likelycaused by charge traps in the active layers employed. The hysteresis issubstantially smaller for the NPD device over the PF-9HK based device,indicating that the hysteresis can be reduced by modification of theactive layer.

In a standard organic TFT the Fermi levels of the source and drainelectrodes are selected to be closely aligned with either the highestoccupied molecular orbital (HOMO) or the lowest unoccupied molecularorbital (LUMO) of the active layer material, leading to a hole carrier(p-type) or an electron carrier (n-type) device, respectively. For somenominal source-drain voltage the gate field modulates the carrierdensity in a thin region at the active-layer/dielectric interface, thusmodulating the current that flows between the source and drain. Incontrast, for embodiments of the invention, the VFET operatesdifferently, as indicated experimentally and by theoretical modeling.While modulation of the carrier density throughout the bulk of theactive layer is indicated by simulation to be possible in the VFETgeometry for very thin active layers, the resulting current modulationis a steep function of the active layer thickness because carriersgenerated nearest the gate electrode screen the gate field from deeperregions of the active layer. For active layer thicknesses >100 nm suchscreening would result in only a small response of the source-draincurrent to the gate field. However, large modulation is observedfor >100 nm active layers with only weak dependence of the gate-fieldlever arm on the active layer thickness. Hence, the gate inducesmodulation of the contact barrier between the nanotubes and the activelayer. Gated nanotube networks are known to demonstratetransconductance, however, achieving appreciable current modulationacross nanotube networks requires the nanotube surface density be verynear the percolation limit such that the threshold percolation pathwaysbridge across semiconducting nanotubes (i.e., the metallic nanotubes,considered alone, must lie below the percolation threshold). When, as inmany embodiments of the invention, the metallic tubes, which aretypically about ⅓ of the nanotubes in the network, are above thepercolation limit, the turn on of the semiconducting nanotubes can onlyaccount for about 67 percent of the modulation observed. Hence, thedevices function as a p-type, Schottky barrier FET in which currentmodulation is due to a gate field induced modulation of the contactbarrier at the nanotube/active layer interface. Modeling of theinjection barrier and the effect of gate field on it indicates this isthe case.

A two-dimensional Poisson equation was solved self-consistently with theequilibrium carrier statistics of the polymer channel and the nanotubecontact for a structure as shown in the inset of FIG. 9. In order tosimplify the modeling and capture the essential physics, the followingconditions were considered: (i) the nanotube network is sparse so thatan individual nanotube is studied for electrostatics in each region;(ii) a 2D cross-section in a vertical plane perpendicular to a nanotubelong axis is simulated; and (iii) the nanotube is an individualsingle-walled metallic tube. A semiconducting nanotube or a small bundlehas a different density of states but does not change the qualitativeresults. FIG. 9 shows the band bending at the nanotube/active layerinterface as a function of the gate field, displaying the barriermodulation.

In embodiments of the invention, the intrinsic low density of states(DOS) for the nanotubes provides one or more advantageous features. Incontrast to metals, which possess a high DOS, the Fermi level of the lowDOS nanotubes can undergo an appreciable shift in response to the gatefield. Hence in addition to the thinning of the contact barrier due tothe gate induced band bending, the barrier height (φ_(bp)) is alsolowered. Literature descriptions of contact barrier modulation atmetallic Schottky contacts are often casually, and incorrectly, labeledas barrier height modulation when what is really meant is barrier widthmodulation due to band bending. The high DOS of metals does not permitthe Fermi level shift necessary for a change in the barrier height. Forreference to the phenomena, a true barrier height modulation can beappreciated from the electrochemically induced barrier height modulationin an air sensitive, polymer/inorganic (poly(pyrrole)/n-indiumphosphide) contact barrier disclosed in Lonergan Science 1997, 278,2103, although this polymer/inorganic system is not metallic. Unlike thesystem of Lonergan, the nanotubes employed in the present invention area true metallic system. Surprisingly the nanotube network based systemsin embodiments of the invention exhibit this novel Schottky contactbarrier modulation in a metallic air stable material. The SWNT networksdescribed herein, which possess this low DOS can, in other embodimentsof the invention, be substituted by a single graphene film,semiconducting nanowires, or conducting nanowires, where, for examplethe nanowires can be silicon.

For the VFET according to embodiments of the present invention, thesemiconductor layer can be selected from the group consisting of: (1) atleast one kind of linearly condensed polycyclic aromatic compound (acenecompound) selected from the group consisting of naphthalene, anthracene,tetracene, pentacene, hexacene, and derivatives thereof; (2) at leastone kind of pigment selected from the group consisting ofcopper-phthalocyanine (CuPc)-based compounds, azo compounds,perylene-based compounds, and derivatives thereof; (3) at least one kindof low-molecular compound selected from the group consisting ofhydrazone compounds, triphenyl methane-based compounds,diphenylmethane-based compounds, stilbene-based compounds, arylvinylcompounds, pyrazoline-based compounds, triphenyl amine derivative (TPD),arylamine compounds, low-molecular weight arylamine derivatives(a.—NPD), 2,2′,7,7′-tetrakis(diphenylamino)-9,9′-spirobifluorene(Spiro-TAD), N,N′-di(1-naphthyl)-N,N′-diphenyl-4,4′-diamonobiphenyl(Spiro-NPB),4,4′4″-tris[N-3-methylphenyl-N-phenylamino]-triphenylamine(mMTDATA), 2,2′,7,7′-tetrakis(2,2-diphenylivinyl)-9,9-spirobifluorence(Spiro-DPVBi), 4,4′-bis(2,2-diphenylvinyl)biphenyl (DPVBi),(8-quininolinolato)aluminum (Alq), tris(8-quinolinolato)aluminum (Alq₃),tris(4-methyl-8quinolinolato)aluminum (Almq₃), and derivatives thereof;and (4) at least one kind of polymer compound selected from the groupconsisting of poly(p-phenylenevinylene) (PPV), polymers containingbiphenyl groups, polymers having dialkoxy groups, alkoxyphenyl-PPV,phenyl-PPV, phenyl/dialkoxy-PPV copolymer,poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV),poly(ethylenedioxythiophene) (PEDOT), poly(styrenesulfonic acid) (PSS),poly(aniline) (PANT), poly(N-vinylcarbazole), halogenatedpoly(N-vinylcarbazole), poly(vinylpyrene), poly(vinylanthracene),poly(alkylfluorenes), pyrene-folmaldehyde resin,ethylcarbazole-folmaldehyde resin, and modifications thereof.

The gate induced band bending and barrier height modulation of nanotubesemployed in embodiments of the invention is shown in FIG. 9, assimulated for a distance of 1 nm from the gate dielectric surface. Thedegree to which the effects occur depends on the distance from the gate.Self screening by the nanotubes reduces the gate lever arm in going fromthe bottom side of a nanotube, nearest the dielectric layer, to its topside. This has the implication that individual nanotubes are preferredover nanotube bundles since the top nanotubes in a bundle are screenedfrom the gate field by underlying nanotubes and participatesubstantially less in the modulation. Nanotube networks in accordancewith embodiments of the invention are formed by a filtration/transfermethod described in Wu et al., Science 2004, 305, 1273, using pulsedlaser vaporization grown nanotubes. All high yield nanotube synthesismethods produce bundles of varying diameter and while ultrasonication insurfactants provides a measure of bundle disassembly. AFM imaging andheight analysis statistics shows these networks to be comprised of abundle distribution ranging in diameters from 1 to 9 nm with a peakcentered at ˜5 nm.

In embodiments of the invention, useful active layer materials have aHOMO that lays within reach of the nanotube Fermi level for rationallyapplicable gate fields. If the active layer HOMO lies above the nanotubeFermi level, the gate field must generate a barrier at the accumulationlayer (anti-barrier) for holes, while if the active layer HOMO liesbelow the nanotube Fermi level the gate must reduce the pre-existingbarrier. Bundled nanotubes in the network can impose more severeconstraints on the active layer materials that will yield usefultransconductance. For active layers possessing a normally on(anti-barrier) band line-up, the top nanotubes in a bundle screened fromthe gate, cannot switch their barriers off. Because the screenednanotubes permit current flow independent of the gate field, suchnormally on devices cannot be turned off effectively, greatly reducingthe on/off ratio. For active layers possessing a normally off bandline-up, the current is switched on by the nanotubes near the bottom ofthe bundles. Although the top nanotubes in the bundles participatelittle in the switching, they do not degrade the on/off current ratio.These inferences are supported by the large on-off ratios observed forPF-9HK and NPD (HOMOs ˜5.6 eV and ˜5.5 eV, respectively, versus acidpurified p-doped SWNT, (work function ˜4.9 eV) in contrast to a pooron/off ratio that is observed with regio-regular poly(3-hexylthiophene)(HOMO ˜5.0 eV) used as active layer.

In embodiments of the invention, individual (unbundled) nanotubes in thesource layer can extend the range of active layers that can be used tomaterials having workfunctions ranging from 4 to 6 eV, which includesmost electroactive polymers and inorganic active layers. Single layergraphene can be used in this application instead of the nanotubes, wherethe single layer graphene can be a continuous layer. The graphene layeris so thin that the change in the graphene Fermi level induced by thegate field causes the barrier height with the active layers to change.

Metals are susceptible to bond formation with active layers that possessa covalent character. Such covalent bonds are implicated in a frequentlyobserved insensitivity of the barrier height formed to the work functiondifference between the metal and semiconductor (Fermi level pinning)Pristine nanotubes, by virtue of their highly passivated graphene-likesurface, do not readily form covalent bonds and leaves the barrierheight predisposed to gate modulation. With respect to defects on thesidewalls of nanotubes, measures can be taken to minimize or healdefects. In embodiments of the invention, an additional advantageafforded by the nanotubes are that the strength with which carbon atomsare held within the nanotube sidewall lattice is such that nanotubes areimpervious to electromigration, which differs from this lifetimelimiting process observed in most metal contact based electronic andelectro-optic devices. Furthermore, the quasi-1D geometry of a nanotubecontact results in a favorable junction electrostatics. The electricfield at the surface of the nanotube is significantly enhanced due toits nanometer-scale radius that further reduces the barrier thickness tofacilitate carrier injection from the nanotube contact into the activechannel.

PF-9HK, initially selected for its low lying HOMO, is also anelectroluminescent polymer. A modification of the top contact to anelectron injecting, small work function metal can create a gated,organic light emitting diode (OLED), where, for example, electronsinjected from the top contact and holes from the nanotubes recombineacross the polymer bandgap to produce light. Such embodiments of theinvention allow control of the emitted light intensity by the appliedgate voltage, which constitutes a vertical organic light emittingtransistor (VOLET). To demonstrate the generality of the design, FIG. 10shows gated light emission in a different system illustrated in FIG. 10Bwhere: Tris (8 -hydroxyquinoline) aluminum (Alq₃) as the photoactivelayer 901, NPD as the hole transport layer 902 and PF-9HK as the gatedhole injecting layer 903. For the device of FIG. 10, to permit lightextraction, the gate electrode 904 is ITO on a transparent substrate 905with a 160 nm, atomic layer deposited, aluminum-titanium oxide (ATO)gate dielectric 906, on which the nanotube network 907 is situated. Fordevices according to embodiments of the invention in which the NPD layer902 directly contacted the nanotubes 907, light is initially emitted butdisplays a quickly decaying luminance in spite of a long term stableoperation of the hole only VFETs using NPD 902 in direct contact withthe nanotubes 907. In an embodiment of the invention, this lifetimeissue is resolved by addition of a PF-9HK as a layer 902 that contactsthe nanotubes 907. In various embodiments, intermediate layers can beused to enhance the ability to modulate the barrier. The device shown inFIG. 10B was fabricated as a hybrid polymer/small molecule device. The200 nm PF-9HK layer 903 was spun onto the nanotubes 907 from toluene,and NPD 902 (100 nm), Alq₃ 903 (50 nm), LiF 908 (1 nm) and Al 909 (100nm) layers were all thermally evaporated. A device was fabricated with apixel of 2×2 mm². In this exemplary embodiment, because of the thinnessof the nanotube source layer, at a drain voltage of −30 V, little gatemodulation occurred, possible because the large source-drain voltageovercomes the major fraction of the barrier, luminance is 540 Cdm⁻², ata current of 17.3 mAcm⁻², which is a current efficiency of 3.1 CdA⁻¹ andis a value comparable to typical ITO anode, NPD/Alq₃ based devices.Bright spots in the pixel zoom were observed for the fabricated devicethat were likely due to particulates that underlie the nanotube networkresulting in a local thinning of the source-drain channel length. Thisis observation was consistent with the principal failure mode of thedevices being direct electrical shorts between the source-drainelectrodes as the electroactive layers thinned.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

1. A light emitting transistor, comprising: a gate electrode forproviding a gate field; a first electrode, wherein the first electrodecomprises a dilute nanotube network; a dielectric layer interposedbetween the gate electrode and the first electrode, a second electrodefor injecting a complementary charge to that injected by the firstelectrode; and an electroluminescent semiconductor layer disposedintermediate the first and second electrodes, wherein the complementarycharges combine to produce photons and wherein charge injection betweenthe first electrode and the electroluminescent semiconductor layer ismodulated by the gate field.
 2. The light emitting transistor of claim1, wherein the nanotube network comprises carbon.
 3. The light emittingtransistor of claim 1, wherein the nanotube network comprises singlewalled carbon nanotubes.
 4. The light emitting transistor of claim 1,wherein the first electrode injects holes and the second electrodeinjects electrons.
 5. The light emitting transistor of claim 1, whereinthe gate electrode is transparent.
 6. The light emitting transistor ofclaim 3, wherein the gate electrode comprises an electrically conductingoxide.
 7. The light emitting transistor of claim 4, wherein the gateelectrode comprises indium tin oxide.
 8. The light emitting transistorof claim 1, further comprising a transparent substrate layer disposedadjacent the gate electrode.
 9. The light emitting transistor of claim1, further comprising a protecting electrode layer disposed adjacent thesecond electrode.
 10. The light emitting transistor of claim 1, whereinboth the first and second electrodes comprise nanotube networks, eachbeing connected to a separate power supply.
 11. The light emittingtransistor of claim 1, wherein the electroluminescent semiconductorlayer possesses a HOMO level between about −3 eV and about −7 eV. 12.The light emitting transistor of claim 1, wherein the nanotube networkcomprises bundled or multiwalled nanotubes and wherein theelectroluminescent semiconductor layer possesses a HOMO level less thana Fermi level of the nanotube network.
 13. The light emitting transistorof claim 1, wherein the electroluminescent semiconductor layer comprisesmethoxy/EtHxOxy poly(phenylene vinylene) (MEH-PPV), oxadiazole pendantpoly(phenylene vinylene), oligo(9,9-di-n-octylfluorene-2,7-vinylene),poly(4-4′-diphenylene diphenylvinylene) (PDPV),poly(9,9-dialkylfluorenes), poly(bis acetylide thiophenes) with BTD orquinoxaline, diphenyloxadiazole pendant polystyrene,5,6,11,12-tetraphenylnaphthacene,bis(4′,6′-Difluorophenylpyridinato)-4,9-bis-[4-(2,2-diphenyl-vinyl)-pheny-l]-naphtho[2,3-c][1,2,5]thiadiazole,4,4′-Bis′2,2′-diphenylvinyl′-1,1′-spirobiphenyl, or factris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a4,4′-N,N′-dicarbazole-biphenyl.
 14. The light emitting transistor ofclaim 1, wherein the second electrode comprises a nanotube network, ametal film, a semiconducting film or a semiconducting nanowire film. 15.The light emitting transistor of claim 1, wherein one or more holetransport layers are disposed between the nanotube network and theelectroluminescent layer.
 16. The light emitting transistor of claim 1,wherein the first electrode comprises a continuous or patterned singlelayer of graphene.
 17. The light emitting transistor of claim 1, whereinthe first electrode comprises a continuous or patterned low carrierdensity and low density of electronic states material, wherein the firstelectrode does not completely screen the gate field from theelectroluminescent semiconductor layer.
 18. A vertical field effecttransistor (VFET), comprising: a gate electrode for providing a gatefield; a first electrode, wherein the first electrode comprises aconductive material having a low carrier density and a low density ofelectronic states; a dielectric layer interposed between the gateelectrode and the first electrode; a second electrode; and asemiconductor layer disposed intermediate the first and secondelectrodes, wherein contact barrier modulation comprises barrier height(φ_(bp)) lowering of a Schottky contact between the first electrode andthe semiconductor by a Fermi level shift.
 19. The VFET of claim 18,wherein the first electrode comprises a dilute nanotube network.
 20. TheVFET of claim 19, wherein the dilute nanotube network comprises singlewalled carbon nanotubes wherein a metallic portion of the nanotubes isof sufficient quantity to be in excess of an electrical percolationthreshold.
 21. The VFET of claim 18, wherein the first electrodecomprises a single layer of graphene, a conducting nanowire network, ora semiconducting nanowire network.
 22. The VFET of claim 18, wherein thesecond electrode comprises a nanotube network, a metal film, asemiconducting film, a conducting nanowire network, or a semiconductingnanowire film.
 23. The VFET of claim 18, wherein the semiconductor layerselected from the group consisting of: (1) at least one kind of linearlycondensed polycyclic aromatic compound (acene compound) selected fromthe group consisting of naphthalene, anthracene, tetracene, pentacene,hexacene, and derivatives thereof; (2) at least one kind of pigmentselected from the group consisting of copper-phthalocyanine (CuPc)-basedcompounds, azo compounds, perylene-based compounds, and derivativesthereof; (3) at least one kind of low-molecular compound selected fromthe group consisting of hydrazone compounds, triphenyl methane-basedcompounds, diphenylmethane-based compounds, stilbene-based compounds,arylvinyl compounds, pyrazoline-based compounds, triphenyl aminederivative (TPD), arylamine compounds, low-molecular weight arylaminederivatives (a.-NPD),2,2′,7,7′-tetrakis(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD),N,N′-di(1-naphthyl)-N,N′-diphenyl-4,4′-diamonobiphenyl(Spiro-NPB),4,4′4″-tris[N-3-methylphenyl-N-phenylamino]-triphenylamine(mMTDATA), 2,2′,7,7′-tetrakis(2,2-diphenylivinyl)-9,9-spirobifluorence(Spiro-DPVBi), 4,4′-bis(2,2-diphenylvinyl)biphenyl (DPVBi),(8-quininolinolato)aluminum (Alq), tris(8-quinolinolato)aluminum(Alq.sub.3), tris(4-methyl-8quinolinolato)aluminum (Almq.sub.3), andderivatives thereof; and (4) at least one kind of polymer compoundselected from the group consisting of poly(p-phenylenevinylene) (PPV),polymers containing biphenyl groups, polymers having dialkoxy groups,alkoxyphenyl-PPV, phenyl-PPV, phenyl/dialkoxy-PPV copolymer,poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV),poly(ethylenedioxythiophene) (PEDOT), poly(styrenesulfonic acid) (PSS),poly(aniline) (PANI), poly(N-vinylcarbazole), halogenatedpoly(N-vinylcarbazole), poly(vinylpyrene), poly(vinylanthracene),pyrene-folmaldehyde resin, ethylcarbazole-folmaldehyde resin, andmodifications thereof.